Traditional fossil fuels have always been the main source of energy, however, long-term exploitation and heavy use results in depletion of resources and also brings about serious environmental pollution. The development and utilization of clean renewable energy sources such as wind, water, solar, and tidal energies have gradually attracted the attention of human society. However, renewable energy sources are difficult to be effectively used by the existing energy management systems due to their inherent intermittence.
Energy storage technology is one of ways to solve such problems. In various kinds of energy storage systems, the all-vanadium redox flow battery (VRB) is an attractive energy storage device. The biggest advantage of VRB is its flexibility—power and energy storage capacity are independent. The power of VRB depends on the number of battery cells and the effective electrode area of battery cells, while the energy storage capacity depends on the concentration of the active material in the electrolyte and the volume of the electrolyte. Each battery cell consists of two electrode chambers (positive and negative electrode chambers) separated by a proton exchange membrane. The electrolyte, that is the sulfate solution of vanadium, is used to store energy. When the electrolyte flows through the battery cell, redox reactions of V(IV)/V(V) and V(II)/V(III) occur in the positive and negative electrode chambers, respectively. The vanadium electrolyte is a vital component of the all-vanadium redox flow battery.
The new vanadium battery stack is generally configured using a mixed vanadium electrolyte with a concentration ratio of V(III) to V(IV) of 1:1, that is, the average valence of vanadium ions in the electrolyte is 3.5. Such electrolyte can be directly added to the positive and negative electrode chambers for use, which is easy to operate. The purity of the vanadium electrolyte plays a crucial role in performances of the battery, and high concentration of impurities in the electrolyte will bring about the following problems: (1) there is a competitive reaction between impurity ions and vanadium ions, which reduces the efficiency of the battery. (2) In the positive electrode chamber, impurity ions are deposited on the graphite felt electrode, which results in the blockage of the gap in the graphite felt and reduction of the specific surface area of the graphite felt, thus affecting charge and discharge efficiencies. (3) In the negative electrode chamber, impurity ions will affect the hydrogen evolution over-potential, and the production of the gas will affect the pressure balance inside the battery. (4) Impurity ions reduce the lifetime of the proton exchange membrane. (5) Impurity ions affect the stability of vanadium ions, leading to premature aging of the electrolyte.
The methods for preparing the VRB electrolyte are as follows: (1) VOSO4 method: U.S. Pat. No. 849,094 discloses a mixed vanadium electrolyte with a concentration ratio of V(III) to V(IV) of 1:1, which is prepared by dissolving VOSO4 in a sulfuric acid solution, and then adjusting the valence state electrochemically. The main problem of this method lies in the more complicated preparation process of VOSO4 and high price, which is not conducive to the large-scale application in VRB. VOSO4 is difficult to be highly purified, thus the electrolyte prepared by such process contains more impurities. Electrochemical treatment is required to adjust the concentration ratio of V(III) to V(IV) to 1:1, so that the average valence of vanadium ions in the electrolyte is 3.5. (2) Chemical reduction method: Chinese patent CN101562256 discloses a mixed vanadium electrolyte of V(III) and V(IV), which is prepared by adding a reducing agent such as oxalic acid, butyraldehyde, etc. to the mixed system of V2O5 and a sulfuric acid solution, and keeping the mixture at 50-100° C. for 0.5-10 hours for chemical reduction. The main problem of the method lies in that it is not easy to achieve the precise control over the degree of reduction. V2O5 prepared by the existing process is difficult to be highly purified, and the electrolyte prepared by such process contains more impurities. Addition of the reducing agent will introduce a new impurity into the vanadium electrolyte system, thereby affecting the purity of the electrolyte. (3) Electrolytic method: International PCT patent AKU88/000471 describes a mixed vanadium electrolyte with a concentration ratio of V(III) to V(IV) of 1:1, which is prepared by adding the activated V2O5 to a sulfuric acid solution, and then performing constant current electrolysis. Preparation of the vanadium electrolyte by the electrolytic method is suitable for large-scale production of the electrolyte, but the process requires a preliminary activating treatment, which needs an additional electrolysis device and consumes electrical energy. Also, there is the problem of the electrolyte containing more impurities. (4) Method by dissolving a low-valence vanadium oxide: Chinese patent CN101728560A discloses that the high-purity V2O3 is used as a raw material and dissolved in 1:1 dilute sulfuric acid at a temperature of 80-150° C. to prepare a solution of V2(SO4)3 used as a negative electrode electrolyte. The main problem of the process lies in that it is operated at a temperature of 80-150° C. (at which temperature the V(III) vanadium ion hydrate is prone to form an oxygen-bridge bond, leading to the production of polycondensation and thus a decreased electrolyte activity), and lacks an activation step. This method can only be used to prepare a negative electrode electrolyte with a narrow application area. Although the industrial high-purity V2O3 used in the patent has a total vanadium content of 67% (corresponding to a purity of 98.5%), it still contains many impurity ions. Chinese patent CN102468509A discloses a method for preparing a vanadium battery electrolyte, which comprises: preparing V2O3 by segmented calcination at 200-300° C. and 600-700° C. with ammonium metavanadate and ammonium bicarbonate as raw materials, dissolving V2O3 in a dilute sulfuric acid and reacting for 5-20 hours at 50-120° C. to obtain a V2(SO4)3 solution, and dissolving V2O5 in the V2(SO4)3 solution and reacting for 1-3 hours at 80-110° C. to obtain a vanadium battery electrolyte with an average vanadium ion valence of 3.5. The V2(SO4)3 solution is prepared as the negative electrode electrolyte in this patent. The main problem of the method lies in the long-time dissolution operation at a higher temperature (at which temperature the V(III) vanadium ion hydrate is prone to form an oxygen-bridge bond, leading to the production of polycondensation and thus a decreased electrolyte activity), and lack of an activation step; and the purity of the electrolyte is not high. Chinese patent CN103401010A discloses a method for preparing an all-vanadium redox flow battery electrolyte, which comprises: reducing V2O5 powder in hydrogen to prepare V204 powder and V2O3 powder, dissolving V204 and V2O3 in the concentrated sulfuric acid respectively to obtain the positive and negative electrode electrolytes of the vanadium battery. The main problem of the patent lies in that no specific reduction process is provided. The V204 powder is prepared by reducing V2O5 in hydrogen, however, in the process, over-reduction or under-reduction is prone to occur and the process only can be achieved by precise control, but the patent does not provide measures about the precise control of reduction. In addition, the purity is low. Chinese patents CN101880059A and CN102557134A disclose a fluidized reduction furnace and reduction method for producing high-purity vanadium trioxide, wherein a heat transfer internal member is added in a fluidized bed to achieve the enhanced heat transfer; and cyclone preheating is used to increase the energy utilization rate and realize the efficient preparation of V2O3. However, since the systems do not have the function of precise control of reduction, the methods described in these two patents are only suitable for the preparation of V2O3 and not suitable for the preparation of other low-valence vanadium oxides.
In summary, there is an urgent need in the art to solve the disadvantages of the process and technology for preparation of the all-vanadium redox flow battery electrolyte, so as to simplify the preparation process, increase the purity of the electrolyte, and improve the simplicity of electrolyte preparation and use.